The objective of this study was to determine if viable Mycobacterium avium subsp. paratuberculosis (MAP) was present in waste milk delivered and fed to calves on California calf ranches. Four calf-raising facilities in the Central Valley of California that fed pasteurized waste milk to calves were enrolled. Pre- and post-pasteurization waste milk samples were cultured for MAP using liquid and solid media over a 5-day period during each of four seasons. Aerobic cultures were performed Show moreThe objective of this study was to determine if viable Mycobacterium avium subsp. paratuberculosis (MAP) was present in waste milk delivered and fed to calves on California calf ranches. Four calf-raising facilities in the Central Valley of California that fed pasteurized waste milk to calves were enrolled. Pre- and post-pasteurization waste milk samples were cultured for MAP using liquid and solid media over a 5-day period during each of four seasons. Aerobic cultures were performed simultaneously to enumerate total bacteria count and evaluate the efficiency of pasteurization which was estimated by the log-reduction of the total number of bacteria. Viable MAP was cultured from 2% of the waste milk samples. Of the three culture-positive samples, two were from pre-pasteurized and one was from post-pasteurized milk samples. The mean total bacterial count for pre- and post-pasteurized waste milk varied from 1.8 x 10(8) to 5.5 x 10(8) colony-forming units (CFU)/mL and 4.9 x 10(5) to 1.1 x 10(8) CFU/mL, respectively, and on average ranches 1, 2, 3, and 4 had, respectively, 3.5-, 3-, 4.7-, and 2.6-log reduction in the number of total bacteria in their waste milk. This is the first study to document results from on-farm pasteurization under field conditions and it indicates the lack of uniformity and adequate controls of the process which could allow the survival of MAP and other pathogens. Calf-raising facilities could benefit from the implementation of standard operating procedures and farm worker training for pasteurization of waste milk. Dairy herds should be aware that placing calves in specialized off-site calf-raising facilities might not eliminate all possible routes of infection of calves with MAP. Show less